<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>BFC</title>
    <style>
        .box {
            border: 1px solid red;
            overflow: auto;
        }

        .box::after {
            content: "\59EC";
            color: #ff00ff;
            width: 20px;
            height: 20px;
        }

        .children {
            background-color: #00ff00;
            float: left;
            width: 50px;
            height: 30px;
            border: 1px dotted #f000ff;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="children">1</div>
        <div class="children">2</div>
        <div class="children">3</div>
        <div class="children">4</div>
        <div class="children">5</div>
    </div>
</body>
</html>